The present invention relates to an image signal processing method and a corresponding apparatus. More particularly, the present invention relates to a synchronous signal determining method and apparatus which can accurately detect a normal image signal despite interference caused by noise.
In general, a television display of an NTSC broadcasting system forms an image frame with two field signals, and one frame is generated from 525 line signals which are contained in an image signal. The image signal also comprises a vertical synchronous signal V-SYNC for synchronizing the two field signals with each other and contains a horizontal synchronous signal H-SYNC for synchronizing the 525 line signals with each other.
In an image signal recording/reproducing apparatus, the synchronous signals V-SYNC and H-SYNC are detected and utilized for various signal processing operations. For example, if a synchronous signal is not detected from an image signal that is input when a television is switched from a broadcasting channel to another broadcasting channel, the apparatus determines that no image signal exists. In such instance, a noise picture, which is normally displayed when a synchronous signal is not detected, is replaced by a blue background picture or another reference picture. Thus, tuning to a channel which is not carrying an image signal is more aesthetically pleasing to a viewer.
In a conventional method for detecting an image signal, the input of an image signal is detected by counting the number of synchronous signals generated during one field or one frame of the image signal or by determining the cycle of the synchronous signal. However, various noises exist in an image signal recording/reproducing apparatus. Thus, when the frequency (or pulse width) of the noises are similar to the frequency (or pulse width) of the synchronous signals, the noises can be misjudged as synchronous signals. (See FIG. 7A). As a result, the method incorrectly detects the presence of an image signal. Similarly, if noise significantly interferes with a valid synchronous signal, the method may incorrectly detect the absence of an image signal.
In order to overcome the above problems, an object of the present invention is to provide a method and apparatus which correctly detect the input of a synchronous signal without error in spite of noise which is similar to the synchronous signal.
In order to accomplish the above and other objects, a method for determining if a synchronous signal is contained in an input image signal is provided. The method comprises the steps of: (a) establishing a horizontal period during which horizontal synchronous signal pulses contained in said input image signal are counted; (b) establishing a vertical period during which vertical synchronous signal pulses contained in said input image signal are counted; (c) establishing a minimum horizontal value and a maximum horizontal value which correspond to a proper number of said horizontal synchronous signal pulses contained in said horizontal period; (d) establishing a minimum vertical value and a maximum vertical value which correspond to a proper number of said vertical synchronous signal pulses contained in said vertical period; (e) counting a first number of said horizontal synchronous signal pulses during said horizontal period to obtain a first counted value; (f) counting a second number of said vertical synchronous signal pulses during said vertical period to obtain a second counted value; and (g) determining if said first counted value is greater than or equal to said minimum horizontal value and less than or equal to said maximum horizontal value; (h) determining if said second counted value is greater than or equal to said minimum vertical value and less than or equal to said maximum vertical value; and (i) determining than an actual synchronous signal is contained in said input image signal if said first counted value is greater than or equal to said minimum horizontal value and less than or equal to said maximum horizontal value and said second counted value is greater than or equal to said minimum vertical value and less than or equal to said maximum vertical value.
In order to further accomplish the above and other objects, a method for determining if a synchronous signal is contained in an input image signal is provided. The method comprises the steps of: (a) establishing a vertical period during which vertical synchronous signal pulses contained in said input image signal are counted; (b) establishing a minimum vertical value and a maximum vertical value which correspond to a proper number of said vertical synchronous signal pulses contained in said vertical period; (c) establishing a minimum noise count value and a maximum noise count value which correspond to a number of noise pulses that are estimated to be similar to said vertical synchronous signal pulses in a noise gate pulse period; (d) generating a noise gate pulse between adjacent pulses of said vertical synchronous signal pulses; (e) counting a first number of said vertical synchronous signal pulses during said vertical period to obtain a first counted value; (f) determining if said first counted value is greater than or equal to said minimum vertical value and less than or equal to said maximum vertical value; and (g) generating a negative determination signal as a current determination signal when said first counted value is less than or equal to said minimum vertical value or is greater than or equal to said maximum vertical value, wherein said negative determination signal indicates that an actual synchronous signal is not contained in said input image signal; (h) counting a second number of said noise pulses during said noise gate pulse period to obtain a second counted value; and (i) generating a positive determination signal as said current determination signal when said first counted value is greater than or equal to said minimum vertical value and less than or equal to said maximum vertical value and when said second counted value is less than said minimum noise count value, wherein said positive determination signal indicates that an actual synchronous signal is contained in said input image signal.
In order to additionally accomplish the above and other objects, an apparatus for determining if a synchronous signal is contained in an input image signal is provided. The apparatus comprises: a vertical synchronous signal separator which separates vertical synchronous signal pulses from composite synchronous signals; a first counter which is operably connected to said vertical synchronous signal separator and which counts a first number of said vertical synchronous signal pulses that exist during a first predetermined period to obtain a first counted value; a first comparator which is operably connected to said first counter and which generates a first detection signal when said first counted value is greater than or equal to a predetermined minimum vertical value and is less than or equal to a predetermined maximum vertical value; a second counter which inputs horizontal synchronous signal pulses contained in said composite synchronous signals and which counts a second number of said horizontal synchronous pulses during a second predetermined period to generate a second counted value; a second comparator which is operably connected to said second counter and which generates a second detection signal when said second counted value is greater than or equal to a predetermined minimum horizontal value and is less than or equal to a predetermined maximum horizontal value; and an operator circuit which inputs said first detection signal and said second detection signal and generates a determining signal based on said first detection signal and said second detection signal.
In order to even further accomplish the above and other objects, an apparatus for determining if a synchronous signal is contained in an input image signal is provided. The apparatus comprises: a vertical synchronous signal separator which separates vertical synchronous signal pulses from composite synchronous signals contained in said input image signal; a first counter which is operably connected to said vertical synchronous signal separator and which counts a first number of said vertical synchronous signal pulses that exist during a first predetermined period to obtain a first counted value; a noise gate pulse generator which inputs said vertical synchronous signal pulses and generates a noise gate pulse between adjacent vertical synchronous signal pulses; a logic switch portion which is operably connected to said noise gate pulse generator and which inputs said composite synchronous signals and said noise gate pulses, processes said composite synchronous signals and said noise gate pulses, and outputs noise pulses; a second counter which is operably connected to said logic switch portion and which counts a second number of said noise pulses to obtain a second counted value; and a comparator which is operably connected to said first counter and said second counter and which generates a current determining signal having a first value indicating that an actual synchronous signal is contained in said input image signal when said first counted value and second counted values have particular values.